COLUMBIA UNIVERSITY RECORD November 19, 1993 Vol. 19 No. 11
SCIENTISTS PUT POLLUTANTS TO USE
Scientists at Columbia's Lamont-Doherty Earth Observatory and the
U.S. Geological Survey have found a new, beneficial use for some
man-made pollutants.
Using trace substances released into the environment by such
things as refrigerators and nuclear explosions, the scientists can
track the underground flow of aquifers that supply water for
drinking and irrigation. The new hydrological tool was reported at
the Geological Society of America's recent annual meeting in
Boston.
The trace substances enter aquifers in extremely small amounts
(too small to be a health hazard) from the atmosphere. By knowing
when they entered the aquifer and in some cases their radioactive
half-lives, the Lamont-Doherty scientists can determine the speed,
as well as the direction, of the water flow.
"Understanding dynamics of groundwater flow is essential for
proper management of this resource," the scientists said at the
GSA meeting.
The new method can help scientists understand how contaminants
spread in an aquifer and how long it takes to flush them out--
information critical for pollution assessment and control.
The techniques can also reveal how quickly an underground water
system is replenished after it is tapped by wells, which can help
prevent overuse, said Peter Schlosser, professor of geological
sciences at Columbia and a senior research scientist at Lamont-
Doherty, Columbia's earth sciences research center in Palisades,
N.Y.
The groundwater research has been conducted by Schlosser, William
Smethie, senior research scientist at Lamont-Doherty, Stefan
Drenkard, a post-doctoral fellow at Lamont-Doherty, and Brenda
Ekwurzel, a Columbia graduate student.
The scientists have transferred to groundwater a technique long
used to study the circulation of ocean currents. They take
advantage of trace substances released into the atmosphere during
the past several decades--tritium, krypton-85 and
chlorofluorocarbons. The substances become dissolved into natural
waters that come in contact with the atmosphere.
Tritium comes from fallout of nuclear bombs tested extensively
from 1952 to 1963, after which atmospheric nuclear tests were
banned. Its atmospheric concentration rose about 1,000 times
during those years, decreasing by 1987 to about five to 10 times
the level in the days before testing. Radioactive tritium is
incorporated in water molecules exchanged between the atmosphere
and soil and subsequently spreads to underground water systems.
Because tritium is essentially non-reactive, it is not absorbed or
degraded as it travels through the aquifer. With a known entry
time and a known half-life, tritium can be used to calculate flow
rates. It has a half-life of 12 years and decays into helium-3,
which can also be traced through the system. The combination of
tritium and helium-3 provides an age of the water since it left
the groundwater table and began flowing underground.
Krypton-85 (KR-85)--another chemically inert, radioactive gas with
a known half-life--can be used similarly. Formed primarily by the
nuclear fission of uranium and plutonium, Kr-85's natural
abundance in the atmosphere is very small and has been completely
overwhelmed by man-made sources--particularly reprocessing of
nuclear fuel for power plants.
From these sources, Kr-85 rapidly mixes throughout the atmosphere
and becomes dissolved in groundwater. Its concentrations have
continually increased since the mid-1950's and are well known from
atmospheric monitoring.
Industrial chlorofluorocarbons (CFCs) are used as refrigerants and
insulators in industrial equipment, air-conditioners,
refrigerators, thermal cups and packing, and began to be released
into the environment in the 1930s and 1940s. Production and
atmospheric records provide a relatively accurate measure of their
atmospheric concentrations over the last 50 years.
The Columbia scientists reconstruct the dynamics of aquifers by
sampling water at several points and various depths. The extremely
small concentrations are measured using Lamont-Doherty's mass
spectrometer for tritium and helium-3; low-level proportional
counters for Kr-85, and a gas chromatograph for the CFCs.
The scientists are using the multi-tracer approach to reduce any
shortcomings an individual measurement may have: Tritium sometimes
cannot be used for waters older than the mid-1960's bomb peak; Kr-
85 is difficult and expensive to collect and analyze, and CFCs can
be absorbed by soil or degraded by bacteria to produce ambiguous
results.
The Columbia scientists collaborate closely with scientists from
the USGS.